on. xi ACROSS THE BORDER 293 



case were new worlds or organisms created, the extension 

 of knowledge being but a consequence of the improved 

 powers of seeing. The limits of visible light-waves 

 have now been extended so that in wireless telegraphy 

 we produce and detect electric waves which do not 

 affect the organ of vision ; and from darkness we 

 derive by X-ray tubes and radium new radiations which 

 light up a land of promise across the border. 



Of all the achievements of science, that which creates 

 the deepest impression upon the minds of most people 

 is wireless telegraphy, the triumphs of which can be 

 traced back to researches carried on in laboratory and 

 study. Its development is due even more to theory 

 than to practical work. When the effects of currents 

 upon magnets, and upon one another, had been deter- 

 mined, science demanded an explanation of them. 

 The effects can be produced in a vacuum, or through 

 glass, wood, or similar substances ; so evidently what 

 seems to be empty space or material bodies must really 

 be filled with something capable of transmitting the 

 electric and magnetic forces. Ampere suggested that 

 the observed facts could be explained by the presence 

 of a universal medium which could convey these forces 

 from one point to another. Henry and Faraday also 

 held this view, and it was developed in detail by a mathe- 

 matical physicist, James Clerk Maxwell, about the 

 middle of the nineteenth century. 



What Maxwell did was to extend by brilliant mathe- 

 matical analysis the conception formed by Faraday as 

 to the propagation of electro-magnetic action by an 

 intervening medium. The theory of " action at a 

 distance " assumed that no medium was actively 

 concerned with the transmission of electric or magnetic 

 forces, whereas Faraday, by experimental researches 



